This was my first computer as well! (age 8?) And I too spent time in class drawing spaceships on graph paper, rather than listening to the teacher. I was certainly obsessed with BASIC programming. I didn't start assembly language until the Apple IIe a few years later.

I'm surprised now to see it was a 16-bit CPU -- I had no idea. I assumed 8-bit as most home computers were (6502, Z80, etc).

Two things I remember:

Saving and loading programs on cassette tapes (where are they? I'm jealous of another poster who still has their TI99/4a).

It was insanely easy to reboot it accidentally -- I had to look it up again but apparently it was by pressing Shift-Q [1]. I lost some work several times, until I got the muscle anti-memory.

[1] https://www.99er.net/994.html